This invention relates to a power supply apparatus for a welder and, more particularly, to such power supply apparatus which is adapted to supply a pulsating current to a welder load when it demands a small current.
For welding small electronic components, laser is frequently used. Laser, however, requires special equipment, and a power supply apparatus for laser is expensive. Recently, therefore, there has been an increasing demand for TIG-welding of such components with a small current.
FIG. 1 shows a block circuit diagram of a prior art TIG welder. An AC voltage from a three-phase commercial AC power supply connected to input terminals 2a, 2b and 2c is applied to a rectifier 4 and is rectified. The output of the rectifier 4 is smoothed by a smoothing capacitor 6 into a DC voltage. The DC voltage is then applied to an inverter 8, where it is converted to a high-frequency voltage. The high-frequency voltage is applied to a transformer 10, where it is voltage-transformed to a high-frequency voltage having a given magnitude. The resultant voltage-transformed, high-frequency voltage is rectified by rectifying diodes 12, 12, and smoothed by a smoothing reactor 14 before it is supplied through output terminals 16a and 16b to a workpiece 18a and a torch 18b. A current detector 20 detects the current supplied to the workpiece 18a and torch 18b, and develops a current-representative signal representing the magnitude of the detected current. The current-representative signal is applied to a control circuit 22. The control circuit 22 receives also a reference current signal from a reference source 24. The control circuit 22 supplies such a control signal to a drive circuit 26 as to make the difference between the current-representative signal and the reference current signal zero (0). In response to the control signal, the drive circuit 26 controls conduction periods of semiconductor switching devices of the inverter 8, so that a load current corresponding to the reference current can be supplied to the workpiece 18a and torch 18b. A high-frequency generator 28 is coupled between the output terminal 16b and the smoothing reactor 14 via a coupling coil 30. The high-frequency generator 28 is used to break the insulation provided by the gap between the workpiece 18a and the torch 18b to thereby cause arcing therebetween to start. The high-frequency generator 28 applies a high-frequency voltage of from 5 kV to 20 kV at a frequency of from 1 MHz to 3 MHz between the workpiece 18a and the torch 18b for a short time for strting the welder. A capacitor 31 is for bypassing a high-frequency signal from the high-frequency generator 28.
Welder users sometimes desire to TIG weld small-sized electronic components with a small current of, for example, about 1 A. However, as is seen from FIG. 2, in order to sustain arcing, a load current of 0.5 A or greater is required. A smaller load current will cause interruption of arcing. Furthermore, a DC current transformer is sometimes used as the current detector 20. In such case, one current transformer must detect a current ranging from a small current of, for example, 1 A, up to a large current of 300 A, for example. At smaller current, such current transformer cannot develop a current-representative signal with linearity. In order to secure the linearity, a Hall-effect current detector may be used, but a Hall-effect current detector tends to produce drifts in the current-representative signal for a small current, and, therefore, such drifts must be controlled. To eliminate the need for such drift control, a series combination of a DC power supply 32, a current-limiting resistor 34 and a switch 36 is connected between the output terminals 16a and 16b. When a small current of, for example, 1 A, is required to flow through the workpiece 18a and the torch 18b, the switch 36 is closed. The DC power supply 32 may be provided, for example, by rectifying the commercial AC power supply, or by rectifying a voltage induced across an additional secondary winding wound on the transformer 10.
However, in order to provide the DC power supply 32, at least a rectifier is additionally required. When the transformer 10 is provided with an additional secondary winding, insulation must be provided between the additional secondary winding and the secondary winding to which the diodes 12 are connected. Thus, an isolation transformer must be used as the transformer 10. Further, when a large current is supplied to the workpiece 18a and torch 18b, the DC power supply 32 need not supply current, and, therefore, the switch 36 is opened. Like this, when a welder is arranged to operate with a small current as well as a large current, a complicated circuit arrangement is required.
A TIG welder may be provided with a sequencing circuit (not shown) and a sequencer control 38. The sequencing circuit is used for initiating operations, such as supplying of inert gas, and the sequencer control 38 is used for controlling the sequencing circuit. The sequencer control 38 detects when current flows through the workpiece 18a and torch 18b, and operates the sequencing circuit. When the load current is small, e.g. of 1 A, a current-representative signal sufficient to initiate the operation of the sequencing circuit is not available.
An object of the present invention is to provide a power supply apparatus for a welder, e.g. a TIG welder, which has a simple circuit arrangement, can perform TIG welding with a small current, and can make, without fail, a sequencing circuit operate even when the load current is small.
A power supply apparatus for a welder according to the present invention has a DC power supply, which, for example, may provide a DC signal by rectifying and smoothing an AC supply or, more specifically, a commercial AC supply. The DC signal from the DC power supply is converted to a high-frequency signal in high-frequency converting means, which may be high-frequency switching means, e.g. an inverter. A transformer transforms the high-frequency signal from the high-frequency converting means. DC converting means converts the transformed, high-frequency signal from the transformer to a DC signal and supplies a load current to a welder load. The load current can be set to a desired value within a range of current values. The DC converting means may be one including rectifying means and smoothing means. Pulse current supplying means is provided, which, when the load current to be supplied to the load from the DC converting means is set to a value smaller than a predetermined current value, supplies a pulse current having a peak value larger than the predetermined current value.
With the above-described arrangement, when the set load current value is below the predetermined current value, a pulse current having a peak value larger than the predetermined current value is supplied. The pulse current can enable, without fail, a sequencing circuit which controls supply of inert gas, for example. In addition, arcing can be sustained as long as the pulse current having a peak value larger than the predetermined current value is flowing.
The welder power supply apparatus may be provided with control means for controlling the high-frequency converting means in such a manner as to make the load current flowing through the welder load have the set current value. The control means receives a load-current representative signal representing the load current. The load-current representative signal may be provided from, for example, load current detecting means. The control means receives also a reference current signal representing the predetermined current value. The control means controls the high-frequency converting means in such a manner as to make the difference between the load-current representative signal and the reference current signal become zero (0). The pulse current supplying means controls the high-frequency converting means in such a manner that the load current flowing through the load can contain the pulse current when the set load current value is smaller than the predetermined current value. For example, when the load current value is set to a value smaller than the predetermined current value, the pulse current supplying means periodically superposes a pulsating signal on the load-current representative signal or on the reference current signal.
With this arrangement, the pulse current can be made to be contained in the load current through the control of the high-frequency converting means, which eliminates the need for providing a dedicated circuit for generating the pulse current in the output of the DC converting means. Thus, the circuitry of the welder power supply apparatus can be simplified.
Alternatively the pulse current supplying means may be arranged to supply the pulse current to the load repetitively at predetermined intervals. In such arrangement, charge-discharge means is provided in the output of the DC converting means. The charge-discharge means is charged with each pulse of the pulse current, and is discharged when no current pulse occurs.
With this arrangement, since the charge-discharge means is discharged when no current pulse occurs, the value of the load current can be maintained large when no current pulse occurs, and, therefore, disruption of arcing can be prevented.
The power supply apparatus may include a sequencer control operable in response to a pulse in the pulse current. Alternatively the sequencer control may be operated by the output of sample-and-hold means which samples and holds the load current.
Since the sequencer control is operated in response to the occurrence of the pulse current, the sequencing circuit controlled by the sequencer control, can be operated, without fail, to cause inert gas to be supplied, for example.